On a BWR fuel bundle, the upper tie plate (UTP) must be set perpendicular to the bundle's fuel rods to allow unrestricted rod growth during irradiation and to ensure an even load on each of the tie rod upper end plugs, which are used to support the bundle during lifting and/or for moving the nuclear fuel bundle around. Currently available UTP perpendicularity gages and set blocks have many issues that cause difficulty in achieving repeatable measurements. The currently available UTP perpendicularity gages yield unrepeatable measurements, even using a single inspector and a single gage.
The currently available UTP perpendicularity gages measure the perpendicularity of the UTP with respect to a single rod within the nuclear fuel bundle. If that fuel rod is bowed, the UTP is repeatedly set incorrectly without taking into consideration the many other fuel rods within the same bundle. The currently available gages are also difficult to use on the bundle sides while the bundle is in the horizontal position.
Additionally, because the single rod being measured is flexible, the inspector can inadvertently rotate or twist the gage handle slightly and the rod the gage rides upon can be moved, yielding an inaccurate perpendicularity reading. Tolerances on the UTP perpendicularity are typically small, e.g. ±0.007 in., and it is easy for two or more people to achieve readings that are different when measuring the same piece. All of these gage issues yield a very technique-driven gage and a high potential for operator error during usage.
Furthermore, the UTP perpendicularity is set while the nuclear fuel bundle is horizontal on the assembly table. In this position, the last support block under the nuclear fuel bundle lies 24-26 inches from the UTP allowing the bundle head to droop a minimum of 0.125 inches due to the weight of the UTP and flexibility of the rods. With the thinner cladding of some nuclear fuel rods, this becomes even more of an issue.